The purpose of the proposed work is to provide a better understanding of the specific role(s) of anterior cingulate cortex (ACC) in selective attention. Of particular interest is the determination of the stages of processing at which the ACC is involved in mediation of conflict resolution (Gehring, 1993) and the degree of conflict required to invoke its activity. The proposed experiments will make use of variants of the Stroop task, as this task provides for direct manipulation of selective attention. Functional MRI will be used to detect task specific changes in the ACC's activity. The first experiment will establish whether or not ACC activity is linked to response interference. This will be accomplished by determining if its activity in the Stroop task is dependent on whether or not the incongruent word names an eligible response. The second experiment will determine if the ACC's activity is dependent on the degree of as semantic conflict present. To do so, increases in ACC activity during the interference condition will be examined a a function of the relative semantic value of the incongruent word. The third experiment will determine if the ACC's activity is dependent on the degree of conflict at the response stage. To do so, increases in ACC activity during the interference condition will be examined as a function of the relative strength of the stimulus to response mappings associated with the incongruent word. The development of a more specific understanding of the ACC's role in selective attention is of importance to the medical field, as recent neuroimaging studies have linked ACC dysfunction to attention deficits observed in clinical populations such as individual suffering from schizophrenia or attention-deficit-hyperactivity-disorder.